Recently, ferromagnetic shape memory alloys have attracted considerable attention because some of them show a giant magnetic field-induced strain of several percent in association with rearrangement of martensite variants. The shape change is due to twinning plane movement, and it does not recover by only removing the magnetic field. In order to recover the strain we need to apply a bias stress or heat up the specimen above the transformation temperature. However, if the martensite phase is induced by magnetic field, and the strain appears in this process, the strain will recover in the field removing process because the reverse transformation occurs in this process. In the present study, therefore, we investigated the effect of magnetic field on the martensitic transformations of Ni_2MnGa,Ni_<2.14>Mn_<0.84>Ga_<1.02> and Ni_<2.14>Mn_<0.92>Ga_<0.94>, which exhibit P (parent phase)-I (intermediate phase)-10M, P-14M-2M and P-2M transformations, respectively. The following results are obtained : (i)the P-I transformation temperature does not change by magnetic field. (ii)the I-10M and the P-14M transformation temperatures decrease under the magnetic field up to 0.8 MA/m and 0.4 MA/m, respectively, and then increase with increasing magnetic fields higher than those fields. (iii)the 14M-2M transformation temperature increases under the magnetic field up to 0.4 MA/m and decreases under the magnetic field up to 0.8 MA/m and then the transformation temperature increases again when magnetic field is applied higher than 0.8 MA/m. (iv)the P-2M transformation temperature increases linearly with increasing magnetic field. These results are well explained by the Clausius-Clapeyron.